Method and apparatus for transmitting an information signal

ABSTRACT

An apparatus and improved method for transmitting an information signal. According to the improved method, an information signal is transmitted under water from a transmitter located at a first place to a receiver located distant therefrom at a second place. In performing the signal transmission, a modulated ultrasound carrier signal is delivered to the water and is formed with a fixed amplitude value in the transmitter. The modulation is performed by pulse duration modulating (with an information signal) the ultrasound carrier signal which, in turn, has a given frequency. The modulated ultrasound carrier signal is then picked up from the water and is demodulated in the receiver in order to recover the information signal. The improvement resides in performing the pulse duration modulation so that the modulated ultrasound carrier signal is modulated in the form of ultrasound pulses having the pulse leading edges thereof modulated with the information signal and having a fixed pulse amplitude below the cavitation limit of the transmission medium. The apparatus performs the improved method.

The invention relates to a method of transmitting an information signalunder water from a transmitter located at a first place to a receiverlocated distant therefrom at a second place, wherein a modulatedultrasound carrier signal delivered to the water is formed with a fixedvalue of the amplitude in the transmitter by modulation of an ultrasoundcarrier signal of a given frequency with the information signal, and themodulated ultrasound carrier signal picked up from the water isdemodulated in the receiver for recovering the information signal, aswell as to an apparatus for carrying out this method.

In a known method of this type (DE-A-2 656 969) the rectangularultrasound carrier signal obtained by a push-pull circuit of monostablemultivibrators is frequency-modulated and, subsequently, converted in afinal stage into a sine-shaped frequency-modulated output signal whichis radiated to the water. The radiated modulated ultrasound carriersignal has a fixed value of the amplitude as a result of its frequencymodulation.

Another known method for underwater signalling between watercraft, inparticular also for telephony operations, is based as well on theproblem that conventional radio signalling techniques cannot be appliedbecause of the bad transmitting capacity of water for electromagneticwaves (J. Matauschek "Einfuhrung in die Ultraschalltechnik" VEB VerlagTechnik Berlin, 2nd edition, August 1961, pages 318 to 320).Particularly, by this method information can be transmitted also betweensubmarine boats travelling underwater. The known method is based onamplitude modulation of the ultrasound carrier signal with theinformation signal to be transmitted. Thus, in this method a certainamplitude swing of the ultrasound carrier signal corresponds to theinformation signal to be transmitted. In this connection the problemarises, however, that cavitation of the ultrasound-conductive medium, i.e. the tearing of it and the formation of small bubbles, occurs above acertain amplitude value of the ultrasound carrier signal, resulting inconsiderable disturbances of the transmission of the information signal.On the other hand, if the amplitude of the ultrasound carrier signalwere restricted to a value below that cavitation limit, only acorrespondingly limited amplitude swing would be available formodulation, so that in particular those portions of the informationsignal that correspond to an amplitude swing that is small in proportionto the maximal amplitude swing show a very bad signal distance fromdisturbances, which gets worse as the distance between the transmitterand the receiver grows.

The use of ultrasound for information transmission is, for instance,also known for television receiver remote-controls. With thisapplication, however, only simple switch signals for channels andcontrol signals for sound and picture adjustment have to be transmitted,so that the demands in connection with the transmission of anyinformation signal whatever with a complicated and differenciatedinformation content do not arise here at all.

The object of the invention is to define a method of the type mentionedat the outset which is improved as far as its range and security againstdisturbances is concerned, and an apparatus for carrying out thismethod, which are in particular suitable for underwater communicationbetween divers.

As far as the method is concerned, this object is achieved according tothe invention in that modulation of the ultrasound carrier signal withthe information signal is performed as pulse duration modulation inwhich the pulse leading edges are modulated with the information signaland the fixed value of the pulse amplitude of the modulated ultrasoundcarrier signal is below the cavitation limit of the medium.

Thus, in the method according to the invention, the cavitation limit ofthe medium `water` which serves for information transmission isapproached only to a point necessary for the desired informationtransmission range. Thereby, an optimum of ultrasonic energy is on theone hand radiated, and the disturbing occurrence of cavitation isavoided on the other hand. Due to the type of modulation applied, wherethe information signal corresponds not to an amplitude of the ultrasoundcarrier signal but to the variation of the ultrasound carrier signalpulses in time which are formed by the pulse modulation, amplitudeeffects which might impair undisturbed transmission of portions of theinformation signal do not play a role. The information to be transmittedis radiated with an intensity below the cavitation limit of the medium,with the same maximum ultrasound intensity being available to allportions of the information signal during transmission. This results ina comparatively large range with high unsensitiveness to disturbances.Pulse modulation, i. e. simply switching on and off the emittedultrasound carrier signal, thereby proves as surprisingly advantageousfor information transmission over large range with small disturbance.

It has turned out that for instance at supersonic frequencies of 100 kHzand more ranges of some kilometers are possible. The method according tothe invention therefore is particularly suitable for underwatercommunication of divers. But this application of the invention is notrestricted to dry-divers in whose diving suits the necessary facilitiesfor carrying out the method can be accommodated. For also wet-divers, i.e. sports divers, can be provided with suitable facilities like anecklace microphone or a mouthpiece adapted for speaking into it so asto be able to establish underwater speech contact by means of the methodaccording to the invention. Another advantage lies in the fact that theangular extension of the sound field emitted by the transmitter caneasily be adjusted by appropriately designing the ultrasound generator.Depending on the desired application, therefore both narrowly limiteddirectional distances between transmitter and receiver and angularlyextended sound fields can be built up. Another example of application ofthe method according to the invention is the transmission of measurementdata delivered by sensors located in water to a processing stationlocated outside the water, for example a drilling platform, yet havingthe receiver of the processing station also located in the water.

In this connection the used type of pulse duration modulation where thepulse leading edges are modulated with the information signal isparticularly advantageous. This measure takes into account that theknown ultrasound generators usually show a certain reverberation periodso that the pulse leading edge of the ultrasound pulse has a betterresolution than the pulse trailing edge.

The various types of pulse modulation are for instance described in E.Holzler and H. Holzwarth "Theorie und Technik der Pulsmodulation",Springer-Verlag 1957. Among these types it is particularly simple tocarry out pulse duration modulation in which the information signal isrepresented by pulses of a given frequency whose pulse durationcorresponds to the scanned amplitude value of the information signal.For example, to form the pulse duration-modulated ultrasound carriersignal it is only necessary to compare the information signal with asaw-tooth signal of a positively given scanning frequency, while fordemodulation in the receiver the electrical signal of the receiver,which corresponds to the pulse duration-modulated ultrasound carriersignal, has only to be smoothed in a low-pass filter for regeneratingthe information signal.

The security of the method according to the invention againstdisturbances is further improved in an advantageous embodiment in thatthe information signal is quantized. As is well-known, correctabilityand security against disturbances of the quantized information signalmodulated then for transmission upon the ultrasound carrier is furtherincreased through quantization, while the minor distortion of theinformation signal caused by the quantization remains negligible.

Although the method according to the invention is suitable for any typeof information signals for the scanning of which a scanning frequencythat is consistent with the time constant of available ultrasoundgenerators is sufficient, the method according to the invention canadvantageously be applied to the case where the information signal is avoice signal. At any rate, ultrasound generators whose time constantallows a scanning frequency sufficient for telephony purposes are easilyavailable.

Thus, the method according to the invention improves the underwatertransmission of messages which is not accessible to radio transmission.As it does not require any complicated appliances and because of itsrange it is particularly suitable for underwater communication betweendivers.

In particular, the scope of the invention also provides that both at thefirs t place and the second place one transmitter and one receiver eachas well as two ultrasound carrier signals of different frequency areprovided, of which the one is used for transmission of information fromthe transmitter of the first place to the receiver of the second placeand the other from the transmitter of the second place to the receiverof the first place. By using two ultrasound carrier signals of differentfrequency thus simultaneous information transmission can take place inboth directions between the first and the second place. In particular,there is the possibility of full-duplex voice communication.

An apparatus for solving the problem that underlies the invention,comprising a transmitter having an ultrasound generator, meansgenerating a drive signal serving for exciting the ultrasound generatorto emit an ultrasound carrier signal of a given frequency and amodulator for generating a modulated fixed-amplitude ultrasound carriersignal by modulation of the drive signal with an information signal froman information signal source, and comprising a receiver having anultrasound pick-up, a demodulator for the output signal delivered by theultrasound pick-up and reproduction means for the demodulated outputsignal, is characterized in that the modulator and the demodulator is apulse modulator and pulse demodulator, respectively, with a constantpulse amplitude modulating and demodulating, respectively, the pulseleading edges, and that the ultrasound generator and/or the ultrasoundpick-up is provided with an ultrasound vibration-attenuating couplinglayer at its ultrasound-radiation or pick-up surface, respectively.

By this attenuation layer, the reverberation of the ultrasoundgenerator, which is due to the time constant, is strongly suppressedwhen its drive signal is switched off, which further improves thescanning resolution as well as the signal/noise ratio.

Besides, the apparatus according to the invention is essentially basedon transmitting the information signal by ultrasound carrier signalmodulation independent of the amplitude. The constant pulse amplitude ofthe ultrasound carrier signal pulses generated under the control by themodulator thus can be adapted to the desired range and can be setaccording to the range requirements up to a maximum value that just doesnot cause cavitation in the medium serving for information transmission.Thus, all those types can be taken into consideration as pulse durationmodulators whose modulation is based on an influence on the rise timeand fall time, respectively, of the leading edges of the pulseamplitudes while the pulse height remains unchanged. In addition, stepsof quantizing the amplitude of the information signal and/or coding maybe provided prior to modulation. Numerous designs of pulse modulators ofthis type with or without quantization or coding, respectively, arewell-known. Such modules are particularly used in digital telephonenetworks where they are commonly known as "CODEC". Thus, the apparatusaccording to the invention can be realized by conventional, easilyavailable modules.

The structure of such pulse length modulators or demodulators,respectively, is very simple. In the first one, merely the informationsignal has to be compared with a sawtooth signal of a fixed andsufficiently high scanning frequency so as to assign to each scanningamplitude of the information signal an ultrasound carrier signal pulseof a time period corresponding to the scanned amplitude. Out of thissequence of transmitted, pulse-duration modulated ultrasound pulses theinformation signal is then regenerated in the pulse duration demodulatorby simple smoothing by means of a low-pass filter.

The advantages of a digital transmission of the information signal areobtained in a suitable development of the apparatus according to theinvention in that an analog/digital converter is connected before thepulse length modulator and a digital/analog converter is connected afterthe pulse length demodulator. So the information signal is transmittedon the transmission path in a form quantized in correspondence with theassigned digital value which may be present in a coded form as is thecase for example in pulse code modulation or also in delta modulation.

An embodiment of the apparatus according to the invention which isparticularly suitable for voice transmission is characterized in thatthe transmitter comprises a microphone for delivering a voice signalrepresenting the information signal. Thus the apparatus can directly beused for telephony operations. Then the reproduction means of thereceiver accordingly comprises an electroacoustic transducer forreproduction of the demodulated output signal which represents a voicesignal.

In particular, it is also provided within the scope of the inventionthat each of a transmitter and a receiver set to frequencies of theultrasound carrier signals different from each other is arranged at theequipment of divers. This enables the divers to communicate with eachother under water by means of the apparatus according to the inventionin full-duplex voice communication. This arrangement of the transmittersand receivers is particularly simple in case of the equipment ofdry-divers where transmitter and receiver can, for example, be arrangedat the helmet or also at other appropriate parts of the equipment. Theapparatus according to the invention is suitable also for sports diverswho work with wet-diving equipment. Here differences exist with regardto the microphone which is connected to the transmitter, it is true. Aswet divers are not prevented from speaking into the mouthpiece of theirbreathing mask, the microphone may, for example, be arranged there.Alternatively, the use of a necklace microphone would be possible, too.

Further features, details and advantages of the invention will followfrom the description hereinbelow and the drawing to which explicitreference is made with regard to any details that are not disclosed inthe text but are essential to the invention. Herein,

FIG. 1a and 1b show a block diagram of an apparatus for transmitting aninformation signal, and

FIGS. 2a -2c illustrate of the operation of a pulse modulator providedin the block diagram of FIGS. 1a and 1b.

According to FIG. 1 (a), a transmitter shown there comprises anultrasound generator 1 which is formed, for example, by a piezoelectricquartz disc provided with electrodes. A means 2 generating an electricdrive signal for the ultrasound generator 1 is connected to theelectrodes, by which the ultrasound generator 1 is excited to emit anultrasound carrier signal, the frequency of which corresponds to thefrequency of the electric drive signal. The electric drive signal ismodulated with an information signal in the manner described hereinafterin detail, so that the ultrasound carrier signal 3 radiated by theultrasound generator 1 shows a certain modulation.

In the illustrated embodiment the information signal is a voice signalgenerated by a microphone 4, which is digitalized after amplification inan amplifier 5 by an analog/digital converter 6. The voice signalquantized in the analog/digital converter 6 is converted in a pulseduration modulator 7 into a pulse duration-modulated ON/OFF signal forthe driver means 2. Thereby the ultrasound carrier signal 3 of theultrasound generator 1 is modulated in the form of ultrasound pulseshaving a pulse duration corresponding to the pulse duration of themodulation signal. In this, the pulse amplitude of the modulatedultrasound carrier signal remains the same and is set to a fixed valuebelow the cavitation limit of the medium in which the modulatedultrasound carrier signal 3 propagates. This avoids the development ofcavitation bubbles in the medium which occur above a certain amplitudevalue and would disturb information transmission.

The process of pulse duration modulation is explained in more detail bymeans of FIG. 2. As follows from FIG. 2 (a), the information signal s₁(t), which for the sake of simplicity is represented here by an analogsignal, i. e. without passing through the analog/digital converter 6, iscompared in the pulse duration modulator 7 with a saw-tooth vibrationwhose frequency is selected to be high enough that a scanning density ofthe information signal s₁ (t) sufficient for the desired transmissionquality is reached. By this scanning, a pulse duration-modulated signalillustrated in FIG. 2 (b) of a constant amplitude is created, with theleading edge of these constant-amplitude pulses each coinciding with thegiven zero-axis crossing time of the leading edge of the saw-toothvibration and the trailing edge of the constant-amplitude pulses eachbeing determined by the point of intersection of the rising edge of thesaw-tooth vibration with the information signal s₁ (t). So the pulseduration of the pulse duration-modulated signal generated according toFIG. 2 (b) just corresponds to the currently scanned amplitude of theinformation signal s₁ (t). The pulse duration-modulated signal of FIG. 2(b) then serves as switch signal for the driver means 2, so that themodulated ultrasound carrier signal 3 shows exactly the same pulseduration modulation.

As can further be seen from FIG. 1 (b), the pulse duration-modulatedultrasound carrier signal 3 transmitted by an ultrasound-transmittingmedium acoustically coupled with the ultrasound generator 1 is picked upby an ultrasound pick-up 8 of a receiver which is acoustically coupledwith the medium, and the ultrasound pick-up 8, just like the ultrasoundgenerator 1, may be formed by a piezoelectric quartz disc provided withelectrodes. The electrical output signal which occurs at the electrodesof the ultrasound pick-up 8 and corresponds to the received modulatedultrasound carrier signal 3 is amplified in a pre-amplifier 9 andsubjected to pulse shaping in a downstream amplifier 10. In ademodulator 11 to which the output signal of the amplifier 10 is applieda digital demodulation signal is gained which corresponds to thedigitalized output signal of the analog/digital converter 6 in thetransmitter, which demodulation signal is converted in a digital/analogconverter 12 into an analog signal corresponding to the informationsignal generated by the microphone 4 of the transmitter. The analogoutput signal of the digital/analog converter 12 is then sufficientlyamplified in a power amplifier 13 so as to be made audible by anelectroacoustic transducer 14, e. g. a loudspeaker or headphones,connected thereto.

The demodulation process is illustrated in FIG. 2 (c) again for thesimpler case where no digitalization is performed. Then merely thepulse-formed signal received from the amplifier 10, which corresponds inits form to the signal of FIG. 2 (b), has to be smoothed by thedemodulator 11 which is designed in the form of a low-pass filter, inorder to recover according to the illustration of FIG. 2 (c) the signalform of the information signal.

As is well-known, ultrasound generators as well as ultrasound pick-upshave a certain time constant, which manifests itself in reverberationafter switching off their excitation. That reverberation limits theresolution between the individual ultrasound pulses. Therefore, anultrasound vibration-attenuating coupling layer may suitably be providedat least between the sound-radiating surface of the ultrasound generator1 and the medium coupled with it. Thereby the undesired reverberationcan largely be suppressed. The suppressing effect may be even increasedin that the ultrasound pick-up is provided with a correspondingattenuating coupling layer on the receiver side, too.

When using the apparatus for full-duplex voice communication betweendivers, each diver is provided with a transmitter and receiver accordingto FIG. 2 (a) and (b), with the ultrasound generator 1 and theultrasound pick-up 8 for example being arranged at the diving helmet orthe diving mask in contact with the surrounding water. The frequenciesof the ultrasound carrier signals of the transmitters are selecteddifferently in order that one can speak simultaneously in bothdirections.

Aside from the aforementioned reverberation period which is due to thetime constant, there is also a transient period because of the timeconstant, which however is smaller than the reverberation period.Therefore, the rising pulse edges, i. e. the leading edges, are bettersuited for modulation than the falling pulse edges. The ultrasoundvibration-attenuating coupling layers simultaneously make attenuation ofthe transient period effects also possible.

List of Reference Numerals

1 ultrasound generator

2 driver

3 ultrasound carrier signal

4 microphone

5 amplifier

6 A/D converter

7 pulse duration modulator

8 ultrasound pick-up

9 pre-amplifier

10 amplifier

11 demodulator

12 D/A converter

13 power amplifier

14 electroacoustic transducer

I claim:
 1. An improved method of transmitting an information signalunder water from a transmitter located at a first place to a receiverlocated distant therefrom at a second place, wherein a modulatedultrasound carrier signal delivered to the water is formed with a fixedamplitude value in the transmitter by pulse duration modulation of anultrasound carrier signal of a given frequency with the informationsignal, and the modulated ultrasound carrier signal picked up from thewater is demodulated in the receiver for recovering the informationsignal, wherein the improvement comprises performing said pulse durationmodulation so that the modulated ultrasound carrier signal is modulatedin the form of ultrasound pulses having only the pulse leading edgesthereof modulated with the information signal such that trailing edgesof the ultrasound pulses remain temporally separated by a constantperiod, said ultrasound pulses having a pulse amplitude of said fixedamplitude value below the cavitation limit of the medium.
 2. The methodaccording to claim 1, characterized in that the information signal isquantized.
 3. The method according to claim 1, characterized in that theinformation signal is a voice signal.
 4. The method according to ofclaims 1, characterized in that both at the first place and the secondplace one transmitter and one receiver each as well as two ultrasoundcarrier signals of different frequency are provided, of which the one isused for transmission of information from the transmitter of the firstplace to the receiver of the second place and the other from thetransmitter of the second place to the receiver of the first place. 5.An apparatus for carrying out the method according to claim 1,comprising a transmitter having an ultrasound generator, meansgenerating a drive signal serving for exciting the ultrasound generatorto emit an ultrasound carrier signal of a given frequency and amodulator for generating a modulated ultrasound carrier signal of afixed amplitude by modulation of the drive signal with an informationsignal from an information signal source, and comprising a receiverhaving an ultrasound pick-up, a demodulator for the output signaldelivered by the ultrasound pick-up and reproduction means for thedemodulated output signal, characterized in that the modulator (7) andthe demodulator (11) is a pulse length modulator and pulse lengthdemodulator, respectively, with a constant pulse amplitude modulatingand demodulating, respectively, the pulse leading edges, and that theultrasound generator (1) and/or the ultrasound pick-up (8) is providedwith an ultrasound vibration-attenuating coupling layer at itsultrasound radiation or pick-up surface, respectively.
 6. The apparatusaccording to claim 5, characterized in that an analog/digital converter(6) is connected before the pulse length modulator (7) and adigital/analog converter (12) is connected after the pulse lengthdemodulator (11).
 7. The apparatus according to claim 5, characterizedin that the transmitter has a microphone (4) for delivering a voicesignal representing the information signal.
 8. The apparatus accordingto of claims 5, characterized in that the reproduction means of thereceiver has an electroacoustic transducer (14) for reproduction of thedemodulated output signal representing a voice signal.
 9. The apparatusaccording to of claims 5, characterized in that each of a transmitter (1to 7) and a receiver (8 to 14) set to frequencies of the ultrasoundcarrier signals different from each other is arranged at the equipmentof divers.